Multimode tracking system utilizing a circular waveguide having slots angularly oriented with respect to the waveguide axis

ABSTRACT

Improvement to systems for determining the position of a telecommunications satellite carrying a radiofrequency beacon transmitting signals of circular polarization of a given direction and in which, whenever the beacon is off the boresight axis of a circular horn antenna, the TM01 mode is excited together with the TE11 mode in the circular horn. The TM01 and TE11 modes are separated by a system including a circular feed waveguide for the horn, a pair of rectangular slots formed therein at two diametrically opposite points in a cross section of the waveguide, said slots having their longer sides respectively perpendicular to the direction of the magnetic field at those points of the TE11 mode having said given direction of polarization, two rectangular section guides connected perpendicularly to the circular waveguide and having their long sides respectively parallel to the long sides of the slots so that the slots excite TE10 mode waves in the rectangular waveguide, a magic T having two input waveguides respectively connected to the two rectangular waveguides, an output guide relating to the antisymmetrical waves in the input guides, at whose output the signal relating to the TM01 mode is collected, and an output guide relating to the symmetrical waves in the input guides and comprising an absorber which absorbs the symmetrical waves.

United States Patent Poitevin [54] MULTIMODE TRACKING SYSTEM UTILIZING ACIRCULAR WAVEGUIDE HAVING SLOTS ANGULARLY ORIENTED WITH RESPECT TO THEWAVEGUIDE AXIS inventor:

Clamart, France [221 Filed: Dec. 15,1969

[21] Appl.No.: 884,814

Jean-Pierre R. Poitevin, 69, rue du Trosy,

[ 51 Feb. 29, 1972 Primary Exaniiner-Richard A. Farley AssistantExaminer-J. M. Potenza Attorney-Abraham A. Saffitz [57] ABSTRACTImprovement to systems for determining the position of atelecommunications satellite carrying a radiofrequency beacontransmitting signals of circular polarization of a given direction andin which, whenever the beacon is off the boresight axis of a circularhorn antenna, the TM. mode is excited together with the TE mode in thecircular horn. The TM and TE modes are separated by a system including acircular feed waveguide for the horn, a pair of rectangular slots formedtherein at two diametrically opposite points in a cross section of thewaveguide, said slots having their longer sides respectivelyperpendicular to the direction of the magnetic field at those points ofthe TE mode having said given direction of polarization, two rectangularsection guidesconnected perpendicularly to the circular waveguide andhaving their long sides respectively parallel to the long sides of theslots so that the slots excite TE mode waves in the rectangularwaveguide, a magic T having two input waveguides respectively connectedto the two rectangular waveguides, an output guide relating to theantisymmetrical waves in the input guides, at whose output the signalrelating to the TM. mode is collected, and an output guide relating tothe symmetrical waves in the input guides and comprising an absorberwhich absorbs the symmetrical waves.

2 Claims, 9 Drawing Figures Pmmgnmzs m2 3. 646, 589

SHEET 2 OF 3 INVENTOR:

Jean-Pierre R. POITEVIN A RN PATENIEDranze m2 3. 646.589

SHEET 3 UF 3 INVENTOR:

J ean-Pierre R POITEVIN The invention relates to microwave multimodesystems for tracking telecommunications satellites without interferingwith the communications signals transmitted by the satellite in the formof-right-hand orleft-hand circularlypolarized electromagnetic waves.

As is known, aground system for tracking a telecommunications satellitetransmitting circularly polarized electromagnetic .waves can have anantenna consisting of a" single frustoconical horn of circular crosssection whichcan be used simultaneously for tracking the satellite andfortransmitting information to the satellite and receiving informationtransmitted by the satellite. The signals picked up by the horn aretransmitted by a circular waveguide to a suitable receivenand thesignals produced by atransmission device are transmitted by thewaveguide and the horn so'as to be radiated or beamed.

It isalso known that, if the plane of the horn opening coincides-withthat of the plane wavetransmitted by the satellite, circularly polarizedwaves of the TE mode are propagated in the circular waveguide.

If the two planes do not coincide and if; the circular waveguide is ofsuitable size, TE mode and TM mode waves are propagated inside it. Thepresence of the TM waves indicates that the antenna axis is out of linewith the satellite. It can be shown that the information about therelative phase and amplitude of the TM mode with respect to the TE modeare connected with the misalignment of the antenna axis with'respe ct tothe direction of the satellite, which is given by a beacon carried bythe satellite, which transmits the signal used by the tracking system.Only waves of the TE mode are used by the receiver connected to thecircular waveguide.

An object of the invention is to provide such a microwave trackingsystem which is capable of extracting from the antennathe.electromagnetic energy of 'the TM mode so as to generate trackingsignals, the information about the phase and amplitude of the TE modeused as reference being taken by simple coupling and filteringat theoutput of the reception amplifier.

Another object of the invention is to design asystemwhich can use theTM. mode to determine the misalignment of the antenna axis withouttakingelectromagnetic energy from the circularly polarized TE waves,without introducing distortion, and without injecting noise into the TEmode waves in anypartof the frequency band used for transmittingtelecommunications information received from the satellite.

Prior art microwave multimode tracking systems comprise a circularwaveguide section disposed between the horn and the feeder" waveguideand formed with two diametrically opposite transverse slots on its wall.The slots are used to couple the waveguide section to a suitablemicrowave system which recombines the coupled waves and which cantherefore, using the symmetry and asymmetry of the modes underconsideration, separate information received on the TM. mode frominformation received on the TE mode.

In such devices, the aforementioned requirements regardingnoninterference with telecommunications information are satisfied byfrequency filtering. To this end, the coupling slots are part offiltering cavities which can facilitate the coupling of the TB and TMmodes at the beacon'frequency-and can eliminate the coupling of thesemodes almost completely at the frequencies of the band usedfortelecommunications.

Since the performance of such filters cannot be improved indefinitely,the device is effective only if the beacon frequency is relativelyremote from the extreme frequencies of the band used fortelecommunications. Furthermore, the beacon frequency cannot be changedwithout adjusting the filtering devices.

The device according to the invention differs from prior art devices inthat the transverse coupling slots are replaced by slots or groups ofslots which are oriented so as to be curs partly in the TM mode andpartly completely decoupled from the circularly polarized principal TE,mode which, as has been stated, conveys information from the satelliteinto the feeder guide.

The resulting decoupling is effective from the wall of the circularguide so that no distortion of the propagation time or mismatchingaffects the propagation of the TB mode. Furthermore, the reinjection ofnoise from the tracking device does not affect the useful TE mode, sincethe reinjection ocin the oppositely polarized TE mode.

The TM mode, on the other hand, is coupled to the tracking device; thecoupling can be increased by using acircular guide near cutoff for theTM mode, resonant slots and cavity resonators. The use of recombinationby symmetry has the following further effects:

a. The TM, mode can be separated from the oppositely polarizedinterfering TE mode which is produced, for example, if there is a defectin the circularity of polarization transmitted by the satellite, inwhich case the interfering TE mode is coupled to the tracking device;

b. Any contribution due to residual coupling of the circularly'polarized TE mode used for reception is eliminated.

Ground telecommunications stations generally use the same antenna fortransmitting and receiving. The system must therefore also besufficiently decoupled from the transmission frequencies not tointroduce excessive mismatching or distortion of the signals transmittedby the station. The decoupling can be achieved by frequency filtering(the action of the aforementioned cavity or slot resonators forincreasing the coupling of the TM mode) or by the association of slotsor systems of slots whose effects are compensated for the TE mode at thetransmission frequencies and are added together at the receptionfrequencies for the TM. mode.

Finally, the system according to the invention is such that the trackingfrequency used can be as near as desired to the frequency band used fortransmitting information from the satellite.

The invention can also, if required, be used to design tracking systemswhich do not need to be adjusted for any change of the beacon frequency.I

A microwave tracking system according to the invention will now bedescribed in detail with reference to the accom panying'drawings, inwhich:

FIG. l'shows a station for tracking satellites and for transmitting andreceiving signals to and from the satellite;

FIG. 2 shows the magnetic field components at two diametrically oppositepoints of a circular waveguide in the case of a right hand or left-handcircularly polarized TE mode wave;

FIG. 3 shows the magnetic field components at two diametrically oppositepoints of a circular waveguide in the case of a TM mode wave;

FIGS. 40 and 4b show the magnetic field components which produce theradiation of the slots cut in a circular waveguide at diametricallyopposite points;

FIGS. 5 and 6 show a microwave system for picking up satellitetracking'signals, according to the invention;

FIG. 7 shows a variant of the invention, and

FIG. 8 shows how a system of regularly spaced, suitably oriented slotscan be used to construct a directional coupler for use inthe couplingsystem according to the invention.

The circularly polarized TE mode obtained by the superposition of tworectilinearly polarized TE modes of equal amplitude, phase quadratureand having directions of polarization which are perpendicular, hascophasal longitudinal and traverse magnetic field components whoseamplitudes are in the same ratio at each instant and at each point onthe surface of the circular guide. One consequence is that the magneticfield of a circularly polarized TE mode has an inclination with respectto the guide axis which remains constant at each instant and at eachpoint on the guide wall.

FIG. 2 shows the magnetic field components in the case where the TE modewave has right-hand circular polarization, at the points M and M on thewall of a circular wave guide I having an axis zz', points M and M beingdiametrically opposed in a single cross section of the guide. At thepoint M, the longitudinal component H, is oriented in the direction ofpropagation when the eomponentli tangential to the guide cross sectionat the point M extends upwards from M in the drawing. The field values HandH are such that the resultant H has a constant inclination a withrespect to the component H whose value depends on the wavelength Acorresponding to the frequency used and to the radius R of guide Theangle na ve! byfiPEEFEEB At the point M, the fields H, and H have thesame values as point M and make the same angle therebetween, but theirorientations are different. The longitudinal component H, extends in theopposite direction to propagation, and the tangential component [1,4 isparallel to H v- The direction of the resultant H' is thereforesymmetrical with the resultant H with respect to the plane of the crosssection of the guide extending through M and M.

If the TE mode wave has a left-hand circular polarization, the componentH, has the same size and direction whereas the componenth is opposite tothe componentl-l corresponding to right-hand circular polarization. Theorientation of the resultant H (left-hand circular polarization) istherefore symmetrical with respect to H with the resultant H shown inFIG. .2.

FIG. 3 shows the field H of a TM mode wave at a point M of thewaveguide 1. The field is a transverse field. At the point M, thetransverse field H is the same but in the opposite direction to H.

.As is well known, a narrow rectangular slot in the wall of a guide isin practice coupled only to the magnetic component parallel to the longside of the slot. Consequently, it is possible to couple modes in acircular guide to an output load, with the exception of a givenparticular mode, by giving to the slot a long side perpendicular to themagnetic field of said particular mode.

FIG. 4a shows a slot of center M formed in guide 1 so that its long sideis perpendicular to the resultant magnetic field H of FIG. 2corresponding to the magnetic field of a left- .hand circularlypolarized TE wave which should not be coupled. In these conditions, theslot will be completely decoupled from the left-hand circularlypolarized TE, wave. On the other hand, rectilinear electric lines offorce E will appear perpendicular to the long sides of slot 10, due tothe presence of the component H, H sin 2a of the magnetic field at thepoint M of the right-hand circularly polarized TE, wave.

If a TM, wave is simultaneously propagated in the guide 1, the presenceof the component H, H cos a at M, due to the TM, wave, will give rise toan electric field 6,, whose lines of force are perpendicular to the longsides of slot 10 and in the same directions as the electric lines offorce 13,.

FIG. 4b shows a slot whose center is a point M. The slot is indicated bybroken lines, since it is seen through the wall of guide I.

The magnetic field component H at M of the left-hand circularlypolarized TE wave which is not to be coupled has a different orientationfrom the component H at the point M, so that the slot 20 must beoriented accordingly.

As in the case of FIG. 4a, the slot is not coupled to the left- .handcircularly polarized TE wave, and the presence of the component H, H sin2a gives an electric field E. which is perpendicular to the two longsides of slot 20 and in the same direction as the (left) vector H. Thepresence of the component H H cos a gives an electric field 6, whoselines of force are perpendicular to the long sides of slot 20 but in theopposite direction to the lines of force of the field E' As can be seen,the electric field components E, and 6,, are in the same direction inslot 10, whereas the corresponding components E, and e, are in oppositedirections in slot 20.

These preliminary considerations are used to design the device forpicking up the tracking signals from a telecommunications satellitewithout interfering with the reception of useful information transmittedby the satellite.

If two rectangular waveguides l1 and 21 whose terminal cross sectionsare formed with slots 10 and 20 (FIGS. 4a and 4b) are welded to thewalls of circular waveguide 1, slots 10 and 20 radiate in therectangular guides and give therein TE mode waves.

If the tracking device is not suitably aimed at the satellite, a TE waveproduced by the TM, mode and a TE wave produced by the TE modes will bepresent in each rectangular waveguide. If the two waves are in phase inone rectangular waveguide, they will be in phase opposition in the otherguide.

if, therefore, the two rectangular waveguides respectively are connectedto the appropriate two arms of a magic T, the TE waves produced by thepresence of the TE waves in the circular guide 1 can easily be separatedfrom the TE waves produced in the waveguide 1 by the TM waves. Since thewaves produced by the TB waves are useless, the can be absorbed in asuitable load in the corresponding arm of the magic T.

On the other hand, the waves produced by the presence of the TM wavescan be used, after their amplitude and phase have been compared withpart of the TE wave received, to operate a rapid-response servomechanismwhich can act on the tracking device motors so as to realign thetracking system with the satellite, as will be seen from the descriptionof FIG. 1.

Referring now to FIG. 5, a circular waveguide portion 1 is formed withtwo narrow, diametrically opposite rectangular slots oriented as shownin FIGS. 4a and 4b. Two rectangular waveguides 11 and 21 are welded tothe walls of guide 1. The centers of the cross sections of therectangular waveguides 11 and 21 coincide with the centers of the slots,and the large faces of waveguides 11 and 21 are parallel to the longsides of slots 10 and 20, as seen in FIG. 6. FIG. 6 shows slot 10, sincethe waveguide 11, seen in end view, has been cut in cross section.

The guide portions 11 and 21 have dimensions suitable for forming cavityresonators, thus increasing the coupling to the beacon frequency. Thecavities are tuned by tuning screws 12 and 22.

13 and 23 are conventional phase-shifting attenuators which can balancethe waves from cavities 11 and 21 before their combination in themagic-T 30. The balancing is brought about by acting on the adjustingscrews 14 and 24 which move the absorbent dielectric strips in thephase-shifting attenuators l3 and 23. Attenuator 13 is connected by a T15 cavity 1 1 and by a bent quide 16 to the am 301 of the magic-T 30.Similarly, the attenuator 23 is connected to cavity 21 by a T 25 and tothe arm 302 of magic-T 30 by the bent quide 26.

When the aforementioned adjustments have been made, the TB wavesproduced by the TM, waves by the misalignment of the device according tothe invention are propagated in arms 301 and 302 of magic-T 30 so thatthe waves in guide 301 are in phase opposition to the waves in guide302. Consequently, the two wave systems combine in the arm 303 ofmagic-T 30.

A conventional transition device 31 closes the arm 303 of magic-T 30, sothat the tracking signals can be transmitted to the tracking receiver bya coaxial cable 32, as shown in FIG. 1.

Waves TE), which are produced by waves TE and are propagated likewise inan'ns 301 and 302 of magic-T 30, combine in arm 304 (shown in end viewin FIG. 5) since the two wave systems are in phase. Arm 304 has amatched termination which absorbs the energy of the useless TE waves.

It should be noted that the mode coupler used in the system according tothe invention eliminates one circularly polarized mode from the wall ofcircular guide 1, so that, unlike the case of prior art devices whicheliminate the coupling by the use of symmetry and antisymmetry, nomismatching or distortion of the group propagation time due to the modecoupler are introduced for the circular polarization mode used for thereception of telecommunications information.

Furthermore, since the inclination of the magnetic field in waveguide 1is independent of the direction of propagation in the guide, thedirection of polarization of the decoupled circular polarized wave (lefthand or right hand) is the same independently of the direction in whichthe waves are propagated inside the circular waveguide 1. If the modecoupler according to the invention is completely decoupled from theleft-hand circularly polarized TE mode, the decoupling persists eitherif the'waves travel from left to right or from right to left in thecircular waveguide l.

A converse result is that the same direction of circular polarization(left hand in FIG. 2) cannot be excited from the coupling slots and 20,either towards the left or the right extremity of the circular guide 1.Consequently, the noise coming from the tracking system will not affectthat amongst the two possible circularly polarized TE, modes selectedfor reception. On the other hand, the coupler is coupled to the TE modeof opposite circular polarization and to the other modes in whichelectromagnetic energy can flow in the guide under consideration.

It should also be noted that, theoretically, wave decoupling cannot beinfinite except at one frequency.

When the device according to the invention is used for frequencies nearthe frequency of maximum decoupling, the expression (1), givenpreviously, shows that:

d(tg a) 2da 1 tg 0: sin 2a h k corresponding to a supplementary powerdecoupling of at least 26 decibels in a relative frequency band ofpercent round the central frequency for waveguide dimensions which willbe given hereinafter.

It will be recalled that the minimum supplementary decoupling of 26decibels characterizes the maximum interference to the propagation of aTE wave used by the telecommunications receiver, but that the use ofsymmetrical recombination considerably reinforces this attenuation for atracking signal taken on the TM, mode.

The aforementioned theoretical considerations are closely confirmed byexperience.

A system for picking up tracking signals according to the inventionwhich gave satisfactory results gave the following performance in thewhole frequency band (3,700-4,200 MHz):

1. The coupling of the TM mode was better than 4 decibels;

2. The decoupling of the left-hand circular TE mode was better than 30decibels; and

3, The relative decoupling at the outlet of arm 303 of magic- T 14between TE waves generated by TM waves and TE waves generated by theright-hand or left-hand circular TE waves was greater than 50 decibels.

These results were obtained with slots mm. long and 2 mm. wide, cut in acircular waveguide 66.5 mm. in diameter and at an angle of 90 -a 58 tothe horizontal.

The cavities 11 and 21 had the following dimensions:

Cross section: 57x22 mm.

Length: 41.8 mm.

The device according to the invention has variant forms.

For example, as FIG. 7 shows, a longitudinal slot lOO parallel to theaxis 22' of the circular waveguide l and a transverse slot 200 can beused for coupling, in which case the sin 2a waveguides 11 and 21 act asa polarization analyser. This is axis 22' which depends not only on therelative values of the transverse and longitudinal components of themagnetic field in the circular waveguide 1, but also on the relativedimensions of the two slots and 2110. It is thus possible, by varyingthe relative dimensions of the slots, to alter the inclination of theresultant magnetic field after coupling, e.g., to 58 with respect to thecross section plane.

If the dimensions of a rectangular waveguide such as 11 are such thatonly the TE mode can propagate inside it, the waveguide is coupled onlyto the component of the transverse magnetic field which is parallel toits long side. it can therefore be used as a rectilinear polarizationanalyser" and can therefore completely eliminate the resultantrectilinear magnetic component corresponding to a given direction ofcircular polarization in the circular waveguide 1. To this end, it issufficient to connect a rectangular guide 11 whose axis is perpendicular to that of the circular waveguide 1 and whose long side isperpendicular to the resultant magnetic field corresponding to thedirection of circular polarization which it is desired not to couple.

The use of cavity resonators and of balancing phase-shifting attenuatorscan likewise be avoided, thus eliminating the need for any adjustmentduring a change of beacon frequency by providing slots such as 10, 20 or100, 200 of dimensions such that they are resonators. The slots, inassociation with other slot resonators in the output guides 11, 21, canform resonating iris bandpass filters covering e.g., the 3,700-4,200MHz. band.

It will also be noted that a number of slots, such as those described inconnection with FIGS. 40 and 4b, can be associated so as to combine thedecoupling effect in a given direction of polarization with adirectivity efl'ect similar to that encountered in microwave systemscalled directive couplers.

FIG. 8 shows a directive coupler between the TM mode, which ispropagated in the circular waveguide 1, and the TE mode of a givenrectangular waveguide 110, which performs the same function as guide 11in FIG. 5. The small surface 111 of the rectangular waveguide isconnected to the circular waveguide l, and slots such as 101, 102 arecut in the surface 111 common to the two guides. If the slots 101, 102are separated by a distance equal to 1 /4 in the two guides (A, beingthe common wavelength in the two guides corresponding to the beaconfrequency), so that the long side a of the cross section of therectangular waveguide 110 conforms to the following relationship:

a z [.3 R R being the radius of the circular waveguide l, the resultingconstruction is a directive coupler between the TM, mode at the input 1of waveguide l and the TE mode at the input 112 of waveguide 110. Theleft-hand circularly polarized TE mode travels through theaforementioned coupler without any interference in either direction ofpropagation.

Referring now to FIG. 1, a communications satellite 52 transmits abeacon wave and a communication wave. The two waves are picked up by aconical horn 51 connected to a circular wave guide 1. A mode coupler 50described in connection with FIG. 5, and a device 53 for convertingcircular to rectilinear polarization, are disposed on waveguide 1 Theoutput waveguide of device 53, in which a transmision wave and areception waves of the TE mode having respectively intersectingrectilinear polarization can be propagated, is connected to atransmitter 55 and to a receiving amplifier 56. Amplifier 56 isconnected to receiver 57 and to a filter 58, which is tuned to thebeacon frequency.

The coaxial cable 32 leaving the mode coupler 50 is connected to thetracking amplifier 59, and the outputs of filter 58 and amplifier 59 areconnected to .an amplitude and phase comparator 60, which in knownmanner supplies a signal showing the amplitude ratio and a signalshowing the phase difference, which are applied to a coordinatestransformer 61. The azimuth and elevation signals generated by thecircuit 61 are applied respectively to motors 62 and 63 which align thehorn 51.

What I claim is:

1. In a system for determining the position of a mobile beacontransmitting signals of circular polarization with respect to a circulartracking horn and in which, whenever the beacon is off the hornboresight axis, the TM mode is excited together with the TE mode in thecircular horn, a device for separating the TM mode wave from the TE modewave having a determined direction of circular polarization, said deviceincluding a circular feed waveguide for said horn, a pair of rectangularslots formed in said circular waveguide at two diametrically oppositepoints in a cross section of the waveguide symmetrically oriented withrespect to said cross section and the longer sides of which areangularly shifted with respect to the axis of said circular feedwaveguide by an angle a given by the formula:

cot a 2. A mode separating device as claimed in claim 1 in which saidcircular waveguide has two sets of said rectangular slots arranged indiametrically opposite pairs and coupled to two rectangular guidesconnected perpendicularly to said circular waveguide, the distancebetween consecutive slots of a set being equal to Ag/4. where Ag, is thecommon wavelength in the circular and rectangular waveguides.

it t I i i

1. In a system for determining the position of a mobile beacontransmitting signals of circular polarization with respect to a circulartracking horn and in which, whenever the beacon is off the hornboresight axis, the TM01 mode iS excited together with the TE11 mode inthe circular horn, a device for separating the TM01 mode wave from theTE11 mode wave having a determined direction of circular polarization,said device including a circular feed waveguide for said horn, a pair ofrectangular slots formed in said circular waveguide at two diametricallyopposite points in a cross section of the waveguide symmetricallyoriented with respect to said cross section and the longer sides ofwhich are angularly shifted with respect to the axis of said circularfeed waveguide by an angle Alpha given by the formula: where lambda isthe used wavelength and R is the radius of said circular waveguide, tworectangular section guides connected perpendicularly to the circularwaveguide and having their long sides respectively parallel to, the longsides of the slots so that the slots excite TE10 waves in therectangular waveguide, a magic T having two input waveguidesrespectively connected to the two rectangular waveguides, an outputguide, relating to the antisymmetrical waves in the input guides, atwhose output the signal relating to the TM01 wave is collected, and anoutput guide, relating to the symmetrical waves in the input guides, andcomprising an absorber which absorbs the symmetrical waves.
 2. A modeseparating device as claimed in claim 1 in which said circular waveguidehas two sets of said rectangular slots arranged in diametricallyopposite pairs and coupled to two rectangular guides connectedperpendicularly to said circular waveguide, the distance betweenconsecutive slots of a set being equal to lambda g/4, where lambda g, isthe common wavelength in the circular and rectangular waveguides.